1. Field of the Invention
This invention relates to white balance correcting devices for video cameras, electronic still cameras or like image sensing apparatuses.
2. Description of the Related Art
In recent years, solid-state image sensors such as a CCD, from the output of which are obtained a luminance signal and color signals, i.e., so-called color solid-state imaging techniques, are coming into ever wider use. With this it is of great importance to insure establishment of the white balance so that regardless of any light source under which a white object is imaged, as the image signal of that object is separated into three primary colors, the ratio of the three color signals, i.e., the R (Red), G (Green) and B (Blue) ones, becomes 1:1:1.
It is known to provide a method of obtaining this white balance by making use of a sensor for detecting the color temperature of the light source, that is, for producing white balance information, as arranged in separation from the sensor which produces image information of the object so that the white balance of the object image can be controlled in accordance with the output of the white balance sensor. Because both sensors differ from each other, the spectral characteristic of the three colors obtained from the white balance sensor does not match with that of the three colors obtained from the solid-state image sensor. This discrepancy must be perfectly removed. Otherwise, the correct white balance could not be made under any type of light source. The color filters to be used with the solid-state image sensor, however, are, for example, ones of four colors including the complementary colors, as is often the case. Therefore, the same dyes as those used in the sensor for white balance cannot be used. Hence, it is quite difficult to match the aforesaid two spectral characteristics with each other. For this reason, it has been difficult to insure that the white balance is maintained with high accuracy over a wide variety of types of light sources.
It is also known to provide another method of obtaining white balance information, which method obviates the necessity of using the additional sensor for white balance by regarding the average value of the image information obtained from the image sensor as white. In this case, there is no problem of the aforesaid discrepancy of the spectral characteristics. But, of the objects which may be encountered, there are some which do not allow the average value to be regarded as white. Thus, the intrinsic color of the object has a large influence on the error of white balance.